1. Field of the Invention
This invention relates to a process for the preparation of chromic acid by the electrolysis of solutions of dichromates and/or monochromates in electrolytic cells in which the anode chamber and cathode chamber are separated by cation exchanger membranes.
2. Description of Related Art
According to CA-A-739 447, the electrolytic preparation of chromic acid (CrO.sub.3) is carried out in an electrolytic cell in which the electrode chambers are separated by a cation exchanger membrane. A solution of an alkali metal dichromate, generally sodium dichromate, or of an alkali metal monochromate or of a mixture of alkali metal dichromate and alkali metal monochromate is introduced into the anode chamber and converted into a solution containing chromic acid by selective transfer of the alkali metal ions into the cathode chamber through the membrane. The concentration of chromic acid and of alkali metal ions in the solution leaving the anode chamber may be adjusted to various values by varying the quantity of alkali metal dichromate introduced into the anode chamber of the cell and the current intensity. The electrolysis is generally operated under such conditions that constant ratios of chromic acid to alkali metal ions are established in continuous operation.
For the production of chromic acid crystals, the solutions formed in the anode chamber of the cell are concentrated by evaporation so that crystallization takes place at, for example, 60.degree. to 100.degree. C. The crystallized chromic acid is then separated, washed and dried.
This process is accompanied by the formation of deposits of compounds of polyvalent ions, in particular of alkaline earth metal compounds, which impair the function of the membrane within a short time until the membrane completely fails. The formation of these deposits is due to the presence of small quantities of polyvalent cations, in particular calcium and strontium ions, in the alkali metal dichromate solutions used as electrolytes, of the kind obtained from the industrial processes described in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Volume A 7, 1986, pages 67 to 97.
It was an object of the present invention to provide a process for the preparation of chromic acid by electrolysis which would be free from the disadvantages described above.
It has surprisingly been found that the aforesaid disadvantages do not occur if the chromic acid content of the solution in the anode chamber of the cell is periodically raised above that of a continuous operating state.
This invention relates to a process for the preparation of chromic acid by the electrolysis of dichromate and/or monochromate solutions in electrolytic cells in which the anode chamber and the cathode chamber are separated by a cation exchanger membrane, characterised in that the chromic acid content of the solution in the anode chamber is periodically increased above that of a continuous operating state.
This increase is preferably brought about by lowering of the rate of throughput of the dichromate and/or monochromate solution through the anode chamber of the cell but may also be brought about by increasing the current intensity up to 3-4 KA/m.sup.2 and/or by an external supply of chromic acid or of chromic acid solution.
In the process according to the invention, the periodic increase in the chromic acid concentration is preferably brought about after 1 to 100 days electrolysis. The point in time chosen for carrying out this measure depends on the concentration of polyvalent cations present in the dichromate and/or monochromate solution. If these cations are present at very low concentrations, the measure may be carried out after more than 100 days. The process according to the invention prevents the formation of deposits and dissolves any deposits already formed so that the service life of the membrane is considerably increased, thereby ensuring prolonged and continuous maintenance of the electrolytic process.
The electrolytic cells used in the examples consisted of anode chambers of pure titanium and cathode chambers of refined steel. Cation exchanger membranes manufactured by DuPont under the name Nafion.RTM. 324 were used as the membranes. The cathodes consisted of refined steel and the anodes of a titanium expanded metal with an electrocatalytically active layer of tantalum oxide and iridium oxide. Such anodes are for example described in U.S. Pat. No. 3,878,083. The distance between the electrodes and the membrane was in all cases 1,5 mm. Sodium dichromate solutions with a content of 800 g/l of Na.sub.2 Cr.sub.2 O.sub.7 .cndot. 2H.sub.2 O and with the contents of impurities indicated in the individual examples were introduced into the anode chambers.
Water was introduced into the cathode chambers at such a rate that a 20% sodium hydroxide solution left the cells. The temperature of electrolysis was in all cases 80.degree. C., and the current density was 3 KA/m.sup.2 of the projected area of the anode and cathode facing the membrane, this area being 11.4 cm . 6.7 cm.